Together, the known adRP genes account for retinal disease in approximately half of the families in this survey, mostly Americans of European origin. Among the adRP genes, IMPDH1, PRPF8, PRPF31, RDS, RHO, and RP1 each accounts for more than 2% of the total; CRX, PRPF3, and RPGR each accounts for roughly 1%. Disease-causing mutations were not found in CA4, FSCN2, NRL, or RP9. Because some mutations are frequent and some regions are more likely to harbor mutations than others, more than two thirds of the detected mutations can be found by screening less than 10% of the total gene sequences. Among the remaining families, mutations may lie in regions of known genes that were not tested, mutations may not be detectable by PCR-based sequencing, or other loci may be involved.

No abstract

Mutations in IMPDH1 account for approximately 2% of families with adRP, and de novo IMPDH1 mutations are also rare causes of isolated LCA. This analysis of the novel IMPDH1 mutants substantiates previous reports that IMPDH1 mutations do not alter enzyme activity and demonstrates that these mutants alter the recently identified single-stranded nucleic acid binding property of IMPDH. Studies are needed to further characterize the functional significance of IMPDH1 nucleic acid binding and its potential relationship to retinal degeneration.

Autosomal dominant retinitis pigmentosa (adRP) is a heterogeneous set of progressive retinopathies caused by several distinct genes. One locus, the RP10 form of adRP, maps to human chromosome 7q31.1 and may account for 5-10% of adRP cases among Americans and Europeans. We identified two American families with the RP10 form of adRP by linkage mapping and used these families to reduce the linkage interval to 3.45 Mb between the flanking markers D7S686 and RP-STR8. Sequence and transcript analysis identified 54 independent genes within this region, at least 10 of which are retinal-expressed and thus candidates for the RP10 gene. A screen of retinal transcripts comparing retinas from normal mice to retinas from crx-/crx- knockout mice (with poorly differentiated photoreceptors) demonstrated a 6-fold reduction in one candidate, inosine monophosphate dehydrogenase 1 (IMPDH1; EC 1.1.1.205). Since many of the genes known to cause retinitis pigmentosa are under CRX control in photoreceptors, IMPDH1 became a high-priority candidate for mutation screening. DNA sequencing of affected individuals from the two American RP10 families revealed a GAC-->AAC transition in codon 226 substituting an asparagine for an aspartic acid in both families. The identical mutation was also found in a British RP10 family. The Asp226Asn missense mutation is present in all affected individuals tested and absent from unaffected controls. The aspartic acid at codon 226 is conserved in all IMPDH genes, in all species examined, including bacteria, suggesting that this mutation is highly deleterious. Subsequent screening of probands from 60 other adRP families revealed an additional family with this mutation, confirming its association with retinitis pigmentosa and the relatively high frequency of this mutation. Another IMPDH1 substitution, Val268Ile, was also observed in this cohort of patients but not in controls. IMPDH1 is a ubiquitously expressed enzyme, functioning as a homotetramer, which catalyzed the rate-limiting step in de novo synthesis of guanine nucleotides. As such, it plays an important role in cyclic nucleoside metabolism within photoreceptors. Several classes of drugs are known to affect IMPDH isoenzymes, including nucleotide and NAD analogs, suggesting that small-molecule therapy may be available, one day, for RP10 patients.

IMPORTANCE X-linked retinitis pigmentosa is a severe inherited retinal degenerative disease with a frequency of 1 in 100 000 persons. Because no cure is available for this orphan disease and treatment options are limited, slowing of disease progression would be a meaningful outcome.OBJECTIVE To determine whether high-dose docosahexaenoic acid (DHA), an ω-3 polyunsaturated fatty acid, slows progression of X-linked retinitis pigmentosa measured by cone electroretinography (ERG). DESIGN, SETTING, AND PARTICIPANTSA 4-year, single-site, randomized, placebo-controlled, double-masked phase 2 clinical trial at a research center specializing in medical retina. Seventy-eight male patients diagnosed as having X-linked retinitis pigmentosa were randomized to DHA or placebo. Data were omitted for 2 patients with non-X-linked retinitis pigmentosa and 16 patients who were unable to follow protocol during the first year. The remaining participants were tested annually and composed a modified intent-to-treat cohort (DHA group, n = 33; placebo group, n = 27).INTERVENTIONS All participants received a multivitamin and were randomly assigned to oral DHA (30 mg/kg/d) or placebo. MAIN OUTCOMES AND MEASURESThe primary outcome was the rate of loss of cone ERG function. Secondary outcomes were rod and maximal ERG amplitudes and cone ERG implicit times. Capsule counts and red blood cell DHA levels were assessed to monitor adherence.RESULTS Average (6-month to 4-year) red blood cell DHA levels were 4-fold higher in the DHA group than in the placebo group (P < .001). There was no difference between the DHA and placebo groups in the rate of cone ERG functional loss (0.028 vs 0.022 log μV/y, respectively; P = .30). No group differences were evident for change in rod ERG (P = .27), maximal ERG (P = .65), or cone implicit time (no change over 4 years). The rate of cone loss (ie, event rate) was markedly reduced compared with rates in previous studies. No severe treatment-emergent adverse events were found.CONCLUSIONS AND RELEVANCE Long-term DHA supplementation was not effective in slowing the loss of cone or rod ERG function associated with X-linked retinitis pigmentosa. Participant dropout and lower-than-expected disease event rate limited power to detect statistical significance. A larger sample size, longer trial, and attainment of a target blood DHA level (13%) would be desirable. While DHA supplementation at 30 mg/kg/d does not present serious adverse effects, routine monitoring of gastrointestinal tolerance is prudent. TRIAL REGISTRATION clinicaltrials.gov Identifier: NCT00100230

In an earlier long-term study of 200 families with adRP, disease-causing mutations were identified in 53% of the families. Mutation-testing by sequencing missed large-scale genomic rearrangements such as insertions or deletions. MLPA was used to identify genomic rearrangements in PRPF31 in five families, suggesting a frequency of approximately 2.5%. Mutations in PRPF31 now account for 8% of this adRP cohort.

Retinitis pigmentosa (RP) refers to a genetically heterogeneous group of progressive neurodegenerative diseases that result in dysfunction and/or death of rod and cone photoreceptors in the retina. So far, 18 genes have been identified for autosomal-dominant (ad) RP. Here, we describe an adRP locus (RP42) at chromosome 7p15 through linkage analysis in a six-generation Scandinavian family and identify a disease-causing mutation, c.449G-->A (p.S150N), in exon 6 of the KLHL7 gene. Mutation screening of KLHL7 in 502 retinopathy probands has revealed three different missense mutations in six independent families. KLHL7 is widely expressed, including expression in rod photoreceptors, and encodes a 75 kDa protein of the BTB-Kelch subfamily within the BTB superfamily. BTB-Kelch proteins have been implicated in ubiquitination through Cullin E3 ligases. Notably, all three putative disease-causing KLHL7 mutations are within a conserved BACK domain; homology modeling suggests that mutant amino acid side chains can potentially fill the cleft between two helices, thereby affecting the ubiquitination complexes. Mutations in an identical region of another BTB-Kelch protein, gigaxonin, have previously been associated with giant axonal neuropathy. Our studies suggest an additional role of the ubiquitin-proteasome protein-degradation pathway in maintaining neuronal health and in disease.

Supplementation of DHA significantly elevated blood DHA levels and reduced the rate of progression in final dark-adapted thresholds and visual field sensitivity. From the relationship between RBC-DHA and the rate of field sensitivity loss, we can extrapolate that an RBC-DHA level of 17% could minimize the decline in field sensitivity. (ClinicalTrials.gov number, NCT00100230.)